What are markush patent claims in United States patent law?

A patent is a set of exclusive rights granted to an inventor for an invention. A patent allows an inventor to prevent other people from making, using, or selling an invention. The heart of a patent is the claims of the patent. The claims of a patent describe the aspects of an invention which are protected by the patent. If an aspect of an invention is described in the patent but not listed in the claims of the patent, than that aspect of the invention is not protected by the patent.

Patent claims have a very specific style. The style used to construct patent claims has developed over the years as United States patent law has evolved. The meaning of words in a patent claim do not always follow the ordinary English language meaning. Even people with strong English language skills have trouble reading patent claims, and more trouble writing patent claims. The reason that patent claims do not always follow the rules of the English language is because so many different types of inventions can be patented. While is may be easy to write patent claims for a simple machine like a bicycle, it is much more difficult to write the patent claims for complex chemicals. As different fields of technology evolve, patent law has evolved to allow inventors to claim their inventions with precise language.

One example of patent claims that have a special meaning are Markush type patent claims. The keyword that identifies a properly drafted Markush claim is the transitional phrase “consisting of”. There are instances where a claim will be treated as a Markush claim even if the phrase “consisting of” is not used but for the purpose of this blog post we will assume the claim was written to intentionally be a Markush claim. The United States Patent and Trademark Office, Manual of Patent Examining Procedure, identifies a Markush claim as a patent claim that is constructed as follows: “selected from the group consisting of A, B, and C.” Markush patent claims may seem completely innocent but they can be a double edged sword.

Markush claims are tricky because normally patent claims are considered open, but Markush claims are considered closed. This distinction is important when the owner of a patent wants to enforce the patent. A patent which uses normal claim language would be infringed by anyone using the invention, even if something was added to the invention. In Markush type claims, the patent is only infringed by using the exact elements recited in the claim.

A recent Federal Circuit case helps illustrate the limiting effect of Markush claims. In MultiLayer Stretch Cling Film v. Berry Plastics (Fed. Cir. 2016) the Federal Circuit was asked to interpret the claim “five identifiable inner layers with each layer being selected from the group consisting of LLDPE, VLDPE, ULDPE, and mLLDPE resins.” The product which allegedly infringed on this patent used layers of other materials, and blends of the four named resins. The Federal Circuit held that the the product did not infringe on the patent. To infringe on the patent a product would need five layers, each layer made from one of the five named resins, and nothing else. Because the product had layers of other material or blends of the five named resins, the product did not infringe.

There are two main reasons to use Markush claims, to limit the number of claims in a patent and avoid prior art. An inventor might want to limit the number of claims in a patent if the inventor is on a budget. The number of claims allowed in a patent are limited to 20 unless additional fees are paid. Using Markush claims allows a broad group of things to be claimed in a single claim. The other reason to use a Markush claim is to avoid prior art. During the course of the patent examination, the patent examiner at the United States Patent and Trademark Office might discover prior art which can only be avoided using a Markush claim.

Drafting patents is an art, even people with great skill in the language the patent is drafted can get trapped by the weird rules of patents. If you have more questions about how to write patent claims it is best to consult with an experienced patent attorney.

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